NASA Technical Reports Server (NTRS) 20070003598: Turbulence and Mountain Wave Conditions Observed with an Airborne 2-Micron Lidar

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by  NASA Technical Reports Server (NTRS)

Turbulence and Mountain Wave Conditions Observed with
an Airborne 2-Micron Lidar
Edward H. Teets, Jr.*, Jack Ehernberger (retired)† and
Rodney Bogue (retired)‡
NASA Dryden Flight Research Center, Edwards, CA, 93523
and
Chris Ashburn§
AS&M, Inc., Edwards, CA, 93523
Joint efforts by the National Aeronautics and Space Administration (NASA), the
Department of Defense, and industry partners are enhancing the capability of airborne wind
and turbulence detection. The Airborne Coherent Lidar for Advanced In-Flight
Measurements (ACLAIM) was flown on three series of flights to assess its capability over a
range of altitudes, air mass conditions, and gust phenomena. This paper describes the
observation of mountain waves and turbulence induced by mountain waves over the
Tehachapi and Sierra Nevada mountain ranges in southern California by lidar onboard the
NASA Airborne Science DC-8 airplane. The examples in this paper compare lidar-predicted
mountain waves and wave-induced turbulence to subsequent aircraft-measured true
airspeed. Airplane acceleration data is presented describing the effects of the wave-induced
turbulence on the DC-8 airplane. Highlights of the lidar-predicted airspeed from the two
flights show increases of 12 m/s at the mountain wave interface and peak-to-peak airspeed
changes of 10 m/s and 15 m/s in a span of 12 s in moderate turbulence.
Nomenclature
ACLAIM = Airborne Coherent Lidar for Advanced In-Flight Measurements
AFB = Air Force base
EDW = Edwards AFB weather observation site
g = acceleration of gravity
ICATS = Information Collection and Transmission System
lidar = light detection and ranging
MHV = Mojave weather observation site
m = meters
mJ = milliJoules
m/s = meters per second
msl = mean sea level
NASA = National Aeronautics and Space Administration
NID = China Lake Naval Weapons Center weather observation site
nsec = nanoseconds
ROS = Rosamond AP156 weather observation site
SNR = signal-to-noise ratio
TAS = true airspeed
TGS = true groundspeed

* Aerospace Engineer, Aerospace Branch, MS 2228, NASA Dryden Flight Research Center.
† NASA Dryden Flight Research Center, retired, AIAA Associate Fellow.
‡ Electronic Engineer, Flight Instrumentation Branch, MS 2306, NASA Dryden Flight Research Center, retired,
AIAA Senior Member.
§ Meteorologist, Aerodynamics Branch, MS 4830D, NASA Dryden Flight Research Center.
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UTC = universal time coordinated
WJF = General Wm. J. Fox Airfield weather observation site
X-acceleration = lateral acceleration
Y-acceleration = longitudinal acceleration
I. Introduction
C
urrent commercial and general aviation aircraft in service lack the capability to detect and mitigate atmospheric
turbulence phenomena. Significant turbulence capable of adversely affecting aircraft ride comfort and
controllability can be generated by convective storms (in visible clouds and in clear air), jet streams (at the
confluence of multiple streams and near boundaries), and mountain waves and rotors (upward propagating and
vortexes).
Turbulence and mountain wave observations were made to improve the detection ahead of the airplane using the
Airborne Coherent Light detection and ranging (Lidar) for Advanced In-Flight Measurements (ACLAIM) system.1
The ACLAIM system was supported by the NASA Aviation Safety Program’s Weather Accident Prevention
(WxAP) Project, Turbulence Prediction and Warning Systems (TPAWS) element. The purpose of the ACLAIM
program is to determine the viability of forward-looking lidar as a sensor for advanced detection of clear-air
turbulence (CAT) and wind shear detection.2 One goal of the program was to develop technology that could "see" in
front of the aircraft with sufficient time to warn of impending CAT and to mitigate the effects on aircraft passengers
and crew.
The ACLAIM was flown on the NASA Airborne Science DC-8 airplane. The ACLAIM team participated in
over 30 DC-8 flights as part of the Convection And Moisture EXperiment 4 (CAMEX-4), Cold Land Processes
Field Experiment (CLPX), and the southern California coastal eddies missions. Figure 1 shows the lidar placement
on the DC-8 for the year 2003 missions. During these missions, the ACLAIM was operated as a "piggyback"
experiment. All flight profiles were driven by the primary mission objectives. On two flight days during the coastal
eddies mission, the ACLAIM lidar became the primary experiment allowing the ACLAIM project personnel to
determine the flight profiles. The data collected from these two flights are the focus of this paper.

Figure 1. The lidar window assembly on the NASA Airborne Science DC-8 airplane; the rotating periscope is
pointing toward the back, in the stowed position (inset).

Two nearly identical ACLAIM systems were built by Coherent Technologies, Inc. of Lafayette, Colorado: one
system for the United States Air Force (USAF) optimized for downward conical scanning and one system for NASA
optimized for forward-looking scanning. The USAF lidar was used in the 1990s by the Airdrop Ballistic Winds
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(ABW) team at the Air Force Research Laboratory (AFRL) at Wright-Patterson Air Force Base (AFB) to flight-test
an innovative wind-measuring instrument that would permit real-time determination of ballistic winds and thereby
improve the accuracy of high-altitude airdrops.3 During the spring 2003 NASA airborne science flight campaigns,
the ACLAIM transceiver built for the AFRL was installed on the DC-8 airplane together with the electronics built
for the NASA ACLAIM system. Table 1 lists the lidar specifications.4

Table 1. Lidar transceiver specifications (based on autumn 2002 tuneup).

Parameter Value
Wavelength, µm 2.0125
Laser pulse energy, mJ 9.3
(out of telescope)
Laser pulse duration, nsec 650
Pulse repetition frequency, Hz 100
Beam diameter, cm 8
Telescope aperture, cm 10
Focal distance, km 2.5
System efficiency, percent 14
Power, W 470

The ACLAIM lidar experiment was an independent experiment operated in conjunction with the NASA
Airborne Science DC-8 coastal eddies mission. The primary goal of the coastal eddies mission was to use surface in
situ measurements and airborne side-looking synthetic aperture radar (AIRSAR) to understand the generation of
spiral-shaped "slicks" on the ocean surface that may result from small eddies typically 5 to 10 km in diameter. These
flight missions took place over the Southern California Bight, a region off the California coast between the Santa
Barbara Channel and the Gulf of Santa Catalina. These missions required clear skies and at least fairly benign wind
conditions to meet the desired objectives. Confined to this restraint, the coastal eddies mission offered little in the
way of turbulence. However, it was during this campaign that the ACLAIM was permitted flight time as the primary
experiment. The lidar team decided to seek and observe mountain waves and turbulence of mountain wave rotors as
the source of disturbed airflow for the lidar evaluation. Mountain waves are the result of restricted vertical motion in
a stable atmosphere caused by the forcing of air over mountain ranges. The vertical motion triggers an oscillating
wave motion similar to waves formed by water flowing over rocks in a fast-running stream.5 A mountain wave rotor
is a turbulent low-level horizontal vortex that forms downstream in close association with mountain waves.6 This
paper details examples of mountain wave and rotor turbulence data collected during these two flights.
II. Field Operations of April 23-28, 2003
The flight of April 23, 2003, dedicated to the lidar team, planned to search for mountain waves being generated
off the southern Tehachapi range, notably the Three Sisters. The Three Sisters are three co-located peaks ranging
from 6300 to 6700 ft (1921 to 2043 m) in elevation. The flight plan called for a series of flightpaths parallel to the
Tehachapi mountain range (perpendicular to the wind field) beginning at 4000 ft (1220 m) mean sea level (msl) and
progressing upward at 2000-ft (610-m) intervals to 8000 ft (2439 m) msl. Both northwesterly and southeasterly
flightpaths were flown at each altitude. A second set of flightpaths were flown perpendicular to the mountain range
(parallel to the wind) beginning at 8000 ft (2349 m) msl with subsequent 2000-ft (610-m) descents to 4000 ft
(1220 m) msl. Like the first set of flightpaths, both east and west flightpaths were flown at each altitude. Figure 2a
shows a closeup view of the flight track used for the April 23 flight.
The flight plan for the mission of April 28, 2003 allowed the ACLAIM team to "search" for turbulence. With a
Pacific storm affecting the west coast of California, the turbulence search was conducted in the Owens Valley,
which is located approximately 100 mi (160 km) north of Edwards Air Force Base (EAFB). The Owens Valley is
well-known for producing numerous and monstrous mountain waves.7 A number of mountain wave research
programs have been conducted in the Owens valley area over the years, two of which were the Sierra Wave project
from 1950 to 19558 and the Terrain-Induced Rotor Experiment (T-REX) in the spring of 2006.9 The current absolute
altitude world record for a sailplane, 49009 ft (14942 m), was achieved in the same vicinity as that in which the
turbulence search was conducted.10 While the DC-8 airplane was at 22000 ft (6707 m) msl, a NASA Dryden Flight
Research Center F-18 pilot on a proficiency flight reported turbulence at 15000 ft (4573 m) msl. The ACLAIM team
then called for the DC-8 airplane to drop to 15000 ft (4573 m) msl to investigate. Once at that altitude, the DC-8
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encountered moderate turbulence associated with a mountain wave rotor. Figure 2b shows a closeup view of the
flight track of the April 28 turbulence encounter.

a) April 23, 2003.

b) April 28, 2003.

Figure 2. Closeup view of the NASA Airborne Science DC-8 flight tracks of April 23 and April 28, 2003.

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III. Data Results
The lidar data display shows radial wind velocity component changes versus distance ahead of the aircraft and is
compared to the aircraft true airspeed minus true groundspeed (TAS – TGS). To perform this comparison, it was
necessary to first translate the aircraft data into the same coordinates as the lidar display. This was accomplished by
using the DC-8 groundspeed and the data sample rate to estimate a distance through space. By translating the DC-8
data versus distance from a given point, the lidar and the aircraft TAS – TGS changes can both be plotted as a
function of distance ahead of the aircraft. With the configuration of the lidar system, this method of comparison will
yield significant lidar-to-TAS correlation if: 1) the aircraft is not maneuvering (i.e. banking or ascending/descending
away from the event), 2) the predicted turbulent event is relatively stationary, 3) the lidar signal-to-noise ratio (SNR)
was sufficient to produce valid data, 4) the turbulent event is not of such small scale that the DC-8 data acquisition
system cannot record it, and 5) the event was somewhat isolated, so that a boundary between perturbed and
unperturbed flow can be distinguished.
During the April 23rd flight, the lidar wave signatures were pronounced. Figure 3 shows examples of lidar
signals compared with aircraft TAS as a function of distance ahead of the airplane. Figure 3a shows the lidar signal
at 21:32:36 UTC approaching the mountain wave at an altitude of 8000 ft (2349 m) msl with the airplane TAS prior
to encountering a mountain wave which is located about 5 km ahead of the aircraft. Gaps in the lidar data are a result
of SNR being below specified limits and are considered unreliable. The ACLAIM system was built on mid-1990s
technology and as a result was underpowered for the long-range detection in clear air that was desired by the project
requirements. Figure 3b, taken 45 s later, shows a well-developed wave with a 12 m/s increase in TAS predicted by
the lidar. Figure 3c, taken at 21:37:15 UTC with the DC-8 at 6600 ft (2012 m) msl and descending on a southeast
heading with a tailwind, presents the mountain wave encounter showing the 12 m/s increase in TAS. The lidar signal
prediction of a 3-km-long mountain wave and the aircraft TAS confirmed this observation. The event was isolated,
and the lidar was able to predict the wave structure out to 4 km (approximately 20 s) ahead of the aircraft.

a) Approaching a mountain wave.

Figure 3. The ACLAIM signatures ahead of the airplane.

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b) Prior to a mountain wave encounter.

c) The entire mountain wave.

Figure 3. Concluded.
The most significant turbulence events from all of the ACLAIM-dedicated flight times occurred during the
April 28 mission. At this time, near 20:29 UTC, the DC-8 airplane was on a north heading at 15000 ft (4573 m) msl
between the Sierra Nevada and Inyo mountain ranges and perpendicular to the wind field (as shown by the flight
track on figure 2b). Figure 4 shows a comparison of the lidar prediction to the observed DC-8 TAS during the
turbulence encounter. This figure shows the lidar at 20:29:11 UTC with signal returns to 3.5 km while in turbulence.
The 15 m/s peak-to-peak (1500 m to 2400 m) corresponds to the 1.3-g excursion shown in figure 6. Some signatures
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of turbulence are observed, but with the prevailing wind being perpendicular to the lidar beam, it is very difficult to
see the extent of the turbulence signatures parallel to the beam. Figures 5 and 6 show DC-8 acceleration data
obtained while flying into and out of the turbulence region. All acceleration data was recorded at 30 samples per
second. Evidence of this rotor is observed in figures 5 and 6 by the abruptness at which the aircraft entered and
exited the moderate-to-strong turbulence. The first and last 25 s of data show very little in the way of significant
accelerations. Figure 5 shows the lateral and longitudinal accelerations as recorded by the DC-8 airplane. Figure 6
shows the vertical accelerations obtained during the turbulence encounter. The g-values (normalized to 9.8 m/s2)
listed on these figures represent the instantaneous g’s as measured by the accelerometers without additional
averaging. The 1.3-g vertical acceleration near 193 s observed in figure 6 corresponds directly to the lidar signatures
in figure 4.

Figure 4. Example of lidar signal (+) compared with airplane TAS (o) as a function of distance ahead of the
airplane.

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Figure 5. Lateral and longitudinal acceleration data for the April 28, 2003 case; time start = 20:26:00, time
stop = 20:30:00.

Figure 6. Vertical acceleration data for the April 28, 2003 case; time start = 20:26:00, time stop = 20:30:00.

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IV. Conclusion
The ACLAIM lidar sensor flown on the NASA Airborne Science DC-8 airplane detected turbulence and
mountain waves at ranges of nearly 4.5 km at 15000 ft (4573 m) msl and nearly 6.0 km at 6000 ft (1829 m) msl
altitude. The longer range of turbulence detection at lower altitude is a result of the presence of more atmospheric
aerosols off of which the lidar energy could reflect. The ACLAIM detected continuous mountain wave signatures as
the aircraft passed through the wave. Turbulence signatures were detected by the ACLAIM system up to entering
the turbulence field, at which point significant maneuvers reduced effective detection. With increasing altitude, the
detection range was only slightly reduced. Significant increases in lidar power will improve the quality of the
returned signal, increasing the range of turbulence detection and thereby increasing the advance time of detection.
References
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